1. Field of the Invention
This invention relates to a combined oil ring of steel used in an internal combustion engine.
2. Description of the Related Art
In general, a two-piece type combined oil ring of cast iron has been normally used in an internal combustion engine, in particular, a diesel engine. In recent years, in place of a cast iron oil ring, a combined steel oil ring has been developed and this has been used in a certain type of engine.
This combined steel oil ring is already described in, for example, U.S. Pat. No. 4,522,412 (Continuation of Ser. No. 436,811) and the gazette of Jap. Pat. Laid-Open No. 61-45172 (Appl. No. 60-167417). The combined oil ring is comprised of a steel oil ring having a generally I-shaped section consisting of upper and lower rigid rails and a thin web connecting the both rails and provided with a plurality of oil holes, and a coil expander forcing the steel oil ring outwardly. The circumferential outer projecting portions of the upper and lower rails of the oil ring have respectively a trapezoidal sectional shape defined by an outer circumferential sliding surface and upper and lower surfaces connecting with the sliding surface. As compared with a cast iron oil ring, a combined steel oil ring has a high strength and a superior conformable characteristic to a cylinder wall, so that it is expected that its practical use will be increased in the future.
In case of the aforesaid combined oil ring, a reduction of width of the outer circumferential sliding surface in the oil ring increases a contact surface pressure with the cylinder and then reduces a thickness of lubricant oil film. In case of the same surface unit pressure as the pressure of conventional ring, its tangential load can be reduced. Since this reduces frictional loss, a reduction of width of the sliding surface is increasingly required more and more in the engines of recent years.
However, as a width of the sliding surface is decreased, a greater anti-wearing characteristic of the outer circumferential sliding surface is required. In general, a sliding surface having a superior anti-wearing characteristic has a lack of toughness. Due to this fact, when a chrome plating or a nitriding treatment or the like was applied to a combined steel oil ring to improve an anti-wearing characteristic, a width of the outer circumferential sliding surface could not be smaller than 0.25 mm. For example, when a thick nitriding treatment which is well known for its superior anti-wearing characteristic is applied to a small circumferential outer projecting portion with a width of the outer circumferential sliding surface smaller than 0.25 mm, the nitriding is advanced from the entire surface of the projecting portion into its inner portion and most of the surface in the section of the projecting portion is nitrided, so that no tough steel bare material is left. Such an oil ring as described above may exhibit chipping off at this part during its machining work or its assemblying work. In this case, a width of the outer circumferential sliding surface of the steel oil ring to which is applied an anti-wearing treatment is normally 0.3 mm to 0.4 mm.